1. Field of the Invention
The present invention generally relates to a pixel clock generation device, a laser scanning device, and an image forming device, and more particularly, to a pixel clock generation device generating a pixel clock, a laser scanning device and an image forming device including the pixel clock generation device.
2. Description of the Related Art
FIG. 1 shows a general configuration of an image forming device, such as a laser printer and a digital composite device. In FIG. 1, a laser light emitted from a semiconductor laser unit 1009 undergoes a scanning by a rotating polygon mirror 1003, and is projected via a scanning lens 1002 on a photo conductor 1001 which is a scanned medium. The projected laser light forms an optical spot on the photo conductor 1001, thereby forming an electrostatic latent image on the photo conductor 1001. Additionally, the polygon mirror 1003 projects the laser light on a photodetector 1004 each time the scanning of one line is finished. When the laser light is projected on the photodetector 1004, the photodetector 1004 converts this laser light into an electric signal, and supplies this electric signal into a phase synchronous circuit 1006. When the electric signal is supplied, the phase synchronous circuit 1006 generates a picture clock (also referred to as a pixel clock) for next one line. Besides, the phase synchronous circuit 1006 is supplied with a high-frequency clock signal from a clock generation circuit 1005; based on the above-mentioned electric signal and the high-frequency clock signal, the picture clock with phase synchronization is generated.
The generated picture clock is supplied to an image processing unit 1007 and a laser drive circuit 1008. The image processing unit 1007 supplies image data to the laser drive circuit 1008 according to the picture clock. The laser drive circuit 1008 drives the semiconductor laser unit 1009 according to the picture clock. Thereby, the electrostatic latent image of the supplied image data is formed on the photo conductor 1001.
Thus, in the image forming device shown in FIG. 1, a light-emission time of a semiconductor laser included in the semiconductor laser unit 1009 is controlled according to the picture clock having a phase set for each line, and the electrostatic latent image on the photo conductor 1001 is controlled accordingly.
A rotating deflector, such as a polygon scanner, is used by a scanning optical system in the above-described configuration. However, the configuration using the rotating deflector has a problem that a distance between a deflective reflection plane and a rotation axis in this deflector varies, and accordingly, a scanning speed of the optical spot (i.e., a scanning beam) scanning a scanned surface becomes irregular. This scanning speed irregularity causes fluctuation in an image, resulting in a deteriorated image quality. Therefore, the scanning irregularity needs correction when a high-quality image is required.
Further, when a multi-beam optical system is used as a light source, a difference in oscillation wavelengths of light-emitting sources of the multi-beam optical system causes exposure displacements, resulting in a difference in scanning widths of optical spots corresponding to the respective light-emitting sources on a scanned medium. Therefore, the scanning width needs correction.
Japanese Laid-Open Patent Application No. 11-167081 and Japanese Laid-Open Patent Application No. 2001-228415 disclose examples of techniques for performing the above-mentioned corrections, such as a method in which an optical spot position is controlled along a scanning line by changing a frequency of a picture clock.
However, the above-mentioned conventional technique of changing the frequency of the picture clock (which is referred to as a frequency modulation method) involves a complicatedly structured picture clock control part. Additionally, this complication increases as a frequency modulation width becomes small. Thus, the frequency modulation method according to the conventional technique has a problem that a fine control cannot be performed.